Proneness of vascular tissue to lipid accumulation and to atherosclerosis has been correlated with its restricted permeability to low density lipoprotein (LDL) and yet no quantitative kinetic measurement of permeability has been made. The objective of this proposal is to use rabbits as experimental animals to determine the rate constants for influx (Kin) and efflux (Kout) of LDL and rabbit serum albumin RSA in the thoracic aorta, a tissue known to be prone to atherosclerosis in this animal as well as in the abdominal aorta. The rate constants are evaluated by a theoretical analysis of the kinetics of the uptake of 125I labeled LDL and serum albumin in the different layers of the arterial tissue. Preliminary experiments using LDL isolated from mildly hypercholesterolemic rabbits, and commercial RSA, show LDL to have a higher Kin compared to RSA; furthermore, the uptake decreases from aortic arch downwards but increases in the abdominal aorta. The LDL used has arginine rich peptide in addition to apo-B. The LDL taken up by the aortic tissue is firmly bound. The decay of arterial radioactivity after multiple injections will be studied for information about accumulation of LDL in the tissue. The dissociation between cholesterol and protein uptake will be studied by double labeled LDL.